Washington — Democrats are bracing to go it alone on the overhaul of the nation’s healthcare system, but the key is first closing their own ranks.

That means closing deep party rifts on issues ranging from a public option and employer mandates to new taxes and affordability for the middle class. Suddenly, securing the votes of liberals like Sen. Jay Rockefeller (D) of West Virginia is as critical as winning over centrist, swing voters like Sen. Ben Nelson (D) of Nebraska.

A key test will be next week’s markup of the Senate Finance Committee’s version of the bill – the last of five panels to produce draft legislation.

“We now have a view of the legislation that the Senate Finance Committee will meld with their HELP bill [Health, Education, Labor, and Pension Committee] in the Senate at some point, and we can have the debate over our differences and build a consensus over where we have agreement, which I think is probably 80 to 85 percent of the bill,” said House Speaker Nancy Pelosi at a press briefing today.

Democrats hope to amplify that agreement and come out of the Finance markup next week with a robust vote. They also need Democrats to hold ranks to block an expected Senate GOP filibuster. But the failure of Sen. Max Baucus (D) of Montana, who chairs the panel, to include a public option is riling liberals, who say it's a nonstarter.

Senator Rockefeller, the No. 2 Democrat on the Finance panel, said on Tuesday that he would vote against any plan that does not include a public option to private insurers.

For months, the focus has been on the bipartisan negotiations in the so-called Gang of Six, three Democrats and three Republicans on the Finance panel. But when Sen. Baucus presented the compromise proposal coming out of those negotiations, he stood alone.

“It’s one of those periodic train wrecks that Congress experiences,” says Ross Baker, a political scientist at Rutgers University in New Brunswick, N.J. “But this is different. Because [healthcare legislation] will be either the crown jewel of the Obama agenda or the collapse of the keystone of his legislative program.”

Some liberal Democrats on the panel aim to use the markup to beef up subsidies for middle-income families to purchase healthcare.

Sen. Ron Wyden (D) of Oregon aims to use the markup to move subsidies for individuals purchasing health insurance up to 400 percent of the federal poverty level. The Baucus plan offers tax credits to individuals and families with incomes between 134 percent and 300 percent.

“The fact that this bill has taken so much heat from both sides of the aisle probably says that there is some pretty good policy there,” says Elizabeth Carpenter, associate director for health policy at the New America Foundation.

Even with the breakdown of negotiations, centrists say it’s too early to count them out.

These include insurance market reforms; prevention and wellness investments; healthcare delivery reforms, including paying for quality rather than quantity; training for healthcare providers and extending tax credits to help pay for insurance.